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Mathematically stated, a collision attack finds two different messages m1 and m2, such that hash(m1) = hash(m2). In a classical collision attack, the attacker has no control over the content of either message, but they are arbitrarily chosen by the algorithm.

More efficient attacks are possible by employing cryptanalysis to specific hash functions. When a collision attack is discovered and is found to be faster than a birthday attack, a hash function is often denounced as "broken". The NIST hash function competition was largely induced by published collision attacks against two very commonly used hash functions, MD5[1] and SHA-1. The collision attacks against MD5 have improved so much that it takes just a few seconds on a regular computer.[2] Hash collisions created this way are usually constant length and largely unstructured, so cannot directly be applied to attack widespread document formats or protocols.

However, workarounds are possible by abusing dynamic constructs present in many formats. In this way, two documents would be created which are as similar as possible in order to have the same hash value. One document would be shown to an authority to be signed, and then the signature could be copied to the other file. Such a malicious document would contain two different messages in the same document, but conditionally display one or the other through subtle changes to the file:

Some document formats like PostScript, or macros in Microsoft Word, have conditional constructs.[3][4] (if-then-else) that allow testing whether a location in the file has one value or another in order to control what is displayed.

TIFF files can contain cropped images, with a different part of an image being displayed without affecting the hash value.[4]

PDF files are vulnerable to collision attacks by using color value (such that text of one message is displayed with a white color that blends into the background, and text of the other message is displayed with a dark color) which can then be altered to change the signed document's content.[4]

An extension of the collision attack is the chosen-prefix collision attack, which is specific to Merkle–Damgård hash functions. In this case, the attacker can choose two arbitrarily different documents, and then append different calculated values that result in the whole documents having an equal hash value. This attack is much more powerful than a classical collision attack.

Mathematically stated, given two different prefixes p1, p2, the attack finds two appendages m1 and m2 such that hash(p1 ∥ m1) = hash(p2 ∥ m2) (where ∥ is the concatenation operation).

In 2007, a chosen-prefix collision attack was found against MD5, requiring roughly 250 evaluations of the MD5 function. The paper also demonstrates two X.509 certificates for different domain names, with colliding hash values. This means that a certificate authority could be asked to sign a certificate for one domain, and then that certificate could be used to impersonate another domain.[5]

A real-world collision attack was published in December 2008 when a group of security researchers published a forged X.509 signing certificate that could be used to impersonate a certificate authority, taking advantage of a prefix collision attack against the MD5 hash function. This meant that an attacker could impersonate any SSL-secured website as a man-in-the-middle, thereby subverting the certificate validation built in every web browser to protect electronic commerce. The rogue certificate may not be revokable by real authorities, and could also have an arbitrary forged expiry time. Even though MD5 was known to be very weak in 2004,[1] certificate authorities were still willing to sign MD5-verified certificates in December 2008,[6] and at least one Microsoft code-signing certificate was still using MD5 in May 2012.

The Flame malware successfully used a new variation of a chosen-prefix collision attack to spoof code signing of its components by a Microsoft root certificate that still used the compromised MD5 algorithm.[7][8]

Many applications of crytographic hash functions do not rely on collision resistance, thus collision attacks do not affect their security. For example, HMACs are not vulnerable.[9] For the attack to be useful, the attacker must be in control of the input to the hash function.

Because digital signature algorithms cannot sign a large amount of data efficiently, most implementations use a hash function to reduce ("compress") the amount of data that needs to be signed down to a constant size. Digital signature schemes are often vulnerable to hash collisions, unless using techniques like randomized hashing.[10]

Note that all public key certificates, like SSL certificates, also rely on the security of digital signatures and are compromised by hash collisions.

The usual attack scenario goes like this:

Mallory creates two different documents A and B, that have an identical hash value (collision).

Mallory then sends document A to Alice, who agrees to what the document says, signs its hash and sends it back to Mallory.

Mallory copies the signature sent by Alice from document A to document B.

Then she sends document B to Bob, claiming that Alice signed the other document (document B). Because the digital signature matches the document hash, Bob's software is unable to detect the modification.

^"Hash Collision Q&A". Cryptography Research Inc. 2005-02-15. Archived from the original on 2008-07-17. Because of the way hash functions are used in the HMAC construction, the techniques used in these recent attacks do not apply

Fast MD5 and MD4 Collision Generators - Bishop Fox (formerly Stach & Liu). Create MD4 and MD5 hash collisions using groundbreaking new code that improves upon the techniques originally developed by Xiaoyun Wang. Using a 1.6 GHz Pentium 4, MD5 collisions can be generated in an average of 45 minutes, and MD4 collisions can be generated in an average of 5 seconds. Originally released on 22Jun2006.